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Connective and supportive tissues (textus connectivus) 81

by vitamin D (1,25-dihydroxycholecalciferol), and inhibi- OSTEOPROGENITOR CELLS
3
VetBooks.ir tion of calcium excretion by the kidney.
Osteoprogenitor cells develop from mesenchymal stem
The hormone calcitonin, secreted by the C-cells of the cells. Typically, these are flattened cells with pale cytoplasm
thyroid gland, stimulates bone production by osteoblasts, and an ovoid to oblong nucleus. Osteoprogenitor cells are
reduces the activity of osteoclasts and promotes the incor- present in the endosteum and periosteum. These mitoti-
poration of calcium into the bone matrix. cally active cells eventually differentiate into osteoblasts.
Somatotropin (growth hormone), adrenocortico- As precursors of osteoblasts, osteoprogenitor cells are
tropic hormone (ACTH), thyroid-stimulating hormone found predominantly in the osteogenic layer of the perios-
(TSH) and male and female reproductive hormones also teum and along vessels of the bone marrow.
have a stimulatory effect on bone growth. In addition,
vitamin C promotes the synthesis of collagen fibres by OSTEOBLAST (OSTEOBLASTUS)
osteoblasts. Vitamin A acts as a regulator in maintaining Unlike their predecessor, osteoblasts do not undergo
an equilibrium between bone production and resorption mitosis. These cells participate in bone formation in the
(Figure 3.30). following ways:
Bone is derived from mesenchymal connective tissue. It
consists of bone cells and bone matrix. · synthesis of type I collagen fibres and non-collage-
nous proteins,
Bone cells · production of glycosaminoglycans/proteoglycans,
Bone cells are comprised of: · participation in mineralisation of bone matrix and
· modulation of osteoclast function.
· osteoprogenitor cells,
· osteoblasts, Analogous to the mechanism of collagen fibril synthesis by
· bone-lining cells, fibroblasts, production of type I collagen fibres by osteo-
· osteocytes and blasts commences within the cell (rER, Golgi apparatus)
· osteoclasts. and is completed in the extracellular space. Osteoblasts
also synthesise ground substance (glycosaminoglycans/
Except for osteoclasts, cells found in bone represent different proteoglycans, other non-collagenous proteins). Together,
functional phases of the same cell type. Osteoprogenitor the irregularly arranged collagen fibres and ground sub-
cells differentiate into osteoblasts (bone-forming cells). stance form the organic, unmineralised bone matrix
Osteoblasts synthesise the organic components of the (osteoid). The fibres are masked by the ground substance.
bone matrix. After mineralisation of the matrix (in which Non-collagenous proteins such as osteocalcin, oste-
osteoblasts also participate), osteoblasts transform into onectin and osteopontin (bone sialoprotein I, BSP-1)
osteocytes. Osteoclasts are responsible for bone resorp- regulate the process of mineralisation. These molecules
tion (Figures 3.31 to 3.33). have a high affinity for calcium ions and hydroxyapatite.

Fibrocytes
Undifferentiated mesenchymal cell Bone marrow

Adipose tissue

Progenitor cell

Macrophage (monocyte) Osteoblast Mineral salt

Proteolytic enzymes
Osteoclast Bone matrix
Bone resorption

3.31 Cellular pathways in bone production and resorption (schematic).

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